First Assessment of Inorganic Nitrogen Deposition Budget Following the Impoundment of a Subtropical Hydroelectric Reservoir (Nam Theun 2, Lao PDR)

Abstract

With around 490 km 2 at full level of operation, the Nam Theun 2 Reservoir (NT2R) is one of the largest hydroreservoir in Southeast Asia. This study presents a first estimation of the atmospheric inorganic nitrogen deposition into the NT2R based on a 2-year monitoring (June 2010 to July 2012) including gas concentrations and precipitation. Dry deposition fluxes are estimated by the inferential method using, on the one hand, surface measurements of gas concentrations (NO 2 , HNO 3 , NH 3 ) from passive samplers and, on the other hand, modeled exchange rates. Wet deposition fluxes are calculated from NH 4 + and NO 3 À concentrations determined in samples of rain from an automatic precipitation collector. The average nitrogen deposition flux is estimated at 1.26 ± 0.14 kgN·ha À1 ·year À1 from dry processes and 5.01 ± 0.92 kgN·ha À1 ·year À1 from wet ones, that is, an average annual total nitrogen flux of 6.27 ± 1.06 kgN·ha À1 ·year À1 deposited at the NT2R with 80% from wet deposition. Before impoundment, the mean N flux has been estimated at 3.42 ± 1.88 kgN·ha À1 ·year À1 for dry deposition and 5.01 ± 2.12 kgN·ha À1 ·year À1 for wet deposition, or a total N deposition flux of 8.43 ± 4.01 kgN·ha À1 ·year À1 over the studied area dominated by forests with little agriculture soil and water surfaces. Thus, the total N deposition over the studied area has been reduced of 26% (or 63% for dry deposition) following the reservoir impoundment based on our working hypothesis.